This invention relates to a starter with a gear reduction mechanism, and more particularly to the structure of an internal gear which is incorporated in a gear reduction mechanism.
FIG. 1 shows a conventional starter with a gear reduction mechanism, such as that shown in Japanese Utility Model Publication No. 58-134056. The numeral 1 indicates a starter with a gear reduction mechanism, and 2 indicates a D.C. motor which is secured to a front bracket 3 with bolts (not shown).
A yoke 4 of the D.C. motor 2 is fixed to a faucet joint portion 3a of the front bracket 3. An intermediate bracket 5 (described hereinafter) made of resin, an isolating plate 6 and a base board 8 for a brush device 7 are integrally secured to the portion 3a of the front bracket 3. The intermediate bracket 5, made of molded synthetic resin which is mainly nylon, consists of a boss portion 5a, a flange 5b, and a cylindrical portion 9a. The flange portion is fitted into the faucet joint portion.
A plurality of teeth are formed on an inner peripheral surface of the cylindrical portion 9a to form an internal ring gear 9. A sleeve bearing 10 is fitted on an inner peripheral surface of the boss portion 5a. An output shaft 11 having a groove 11a formed at one end thereof is rotatably supported on the sleeve bearing. On an outer peripheral surface of the output shaft 11 is formed a helical spline 11b which engages an overrunning clutch (not shown). At the rear end portion of the output shaft 11 a flange 12 is fixed, in which supporting pins 13 are planted. The supporting pins 13 support planetary gears 14 through a sleeve bearing 15. The planetary gears 14 mesh with the internal ring gear 9 and with a spur gear 18, which is provided at the front end portion of a rotary shaft 17 of an armature 16.
The planetary gear reduction mechanism operates through the gear train constructed as described above. A sleeve bearing 19 for rotatably supporting the front end of the rotary shaft 17 of the armature 16 is provided on an inner peripheral surface of the groove 11a of the output shaft 11. A steel ball 20 is inserted into the space between the front end of the rotary shaft 17 and the output shaft 11, in order to absorb the load applied to the rotary shaft 17 and the output shaft 11. A commutator 21 is fitted on the rotary shaft 17, and has one end electrically connected to an armature coil 22 which is wound around an armature core 23. Brushes 24 supported by brush holders 25 are in slidable contact with the commutator 21 under a force provided by a spring 26. Poles 27 made of permanent magnetic materials such as ferrite are mounted on an inner peripheral surface of the yoke 4, to which a grommet 28 is attached.
The operation of the conventional starter thus constructed now will be described. The brushes 24 are electrically connected to an external power source (not shown), and the armature current is supplied to the armature coil 22 through the brushes 24 and the commutator 21. Thus the armature 16 is rotated through magnetic fields generated by the poles 27. The rotating force of the armature 16 is transmitted to the output shaft 11, with the speed of the rotary shaft 17 being reduced by the planetary gears 14.
Consequently, an internal combustion engine (not shown) is rotated by the overrunning clutch (not shown) which turns with the output shaft 11 by means of a spline (not shown) which connects the output shaft with the clutch. The internal ring gear 9 undergoes stress caused by the meshing of the internal ring gear 9 with the planetary gears 14. Consequently, it is necessary to reinforce the cylindrical portion 9a of the internal ring gear 9 in order for the gear to have the same strength as a gear formed of metal, since the internal ring gear 9 is formed of resin.
To meet the strength requirement, the cylindrical portion 9a has had to be made relatively thick, so that the size and cost of the internal ring gear 9 have been substantial.
Furthermore, the toothed portion of the internal ring gear 9 are deformed by a sink phenomenon which occurs during the molding process. In addition, the internal ring gear 9 is deformed and is sometimes destroyed, since the pressure is concentrically applied to the toothed portion of the internal ring gear 9 which is meshed with the planetary gears 14. For example, when three planetary gears are arranged at equal angular intervals, the internal ring gear 9 is deformed into a triangular shape.